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Critical, CVSS 9.1amended

yayson: Prototype pollution in Store/LegacyStore deserialization

CVE-2026-61534GHSA-325j-mg25-8q58

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (yayson 4.3.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm yayson

    Affected
    <= 4.2.0
    Fixed in
    4.3.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
9.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
EPSS
  • CVE-2026-61534: 0.84% estimated exploitation activity in the next 30 days; percentile 57.91 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary Store/LegacyStore key internal lookup tables by the type, id, and relationship names from a JSON:API document. Because these were plain objects, a document with type: "proto" writes onto Object.prototype, polluting every object in the process. Severity Suggested CVSS v3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H (8.1). The guaranteed impact is process-wide DoS / logic corruption; escalation to authorization bypass or RCE is dependent on gadgets in the consuming application. Affected / Patched - Affected: <= 4.2.0 (verified in 3.0.0 and 4.2.0; all 3.x and 4.x). - Patched: 4.3.0. Details type is a string value, untouched by JSON.parse, and is used directly as an object key: The attacker controls the polluted key (id) and value (the model, populated from attributes). Pollution persists for the process lifetime. The malicious type can also arrive via an included resource referenced by a relationship, bypassing any data.type allow-list. LegacyStore is additionally reachable when a…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-325j-mg25-8q58

  2. Revision 2 · amended ·

    Amended from osv record GHSA-325j-mg25-8q58

  3. Revision 1 · created ·

    Published from ghsa record GHSA-325j-mg25-8q58

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-61534

Advisory: yayson: Prototype pollution in Store/LegacyStore deserialization
IDs: CVE-2026-61534, GHSA-325j-mg25-8q58
Record: https://patchnext.com/advisories/cve-2026-61534 (revision 3, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: yayson 4.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm yayson: affected <= 4.2.0; fixed in 4.3.0

## Suggested inspection (read-only)
- `npm ls yayson --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on yayson.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-325j-mg25-8q58 (GitHub-reviewed advisory GHSA-325j-mg25-8q58 lists affected ranges: npm yayson >=0 <=4.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-325j-mg25-8q58 (OSV record GHSA-325j-mg25-8q58 lists affected ranges: npm yayson >=0 <4.3.0 (fixed))

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